is it? evaluating the risk eating bacon sandwiches is bad for you!

Is it?Is it?

Evaluating the Evaluating the riskrisk

Eating bacon Eating bacon sandwiches sandwiches is bad for is bad for

you!you!

If you eat a bacon sandwich every day, your risk of getting bowel cancer at some point in your life is increased by 20%.

Eating bacon Eating bacon sandwiches is sandwiches is bad for you!bad for you!

If you eat a bacon sandwich every day, your risk of getting bowel cancer at some point in your life is increased by 20%.

Sounds Sounds bad! bad!

What do What do you you

think?think?

Eating bacon Eating bacon sandwiches is sandwiches is bad for you!bad for you!

If you eat a bacon sandwich every day, your risk of getting bowel cancer at some point in your life is increased by 20%.

Sounds Sounds bad! bad!

What do What do you you

think?think?

Eating bacon Eating bacon sandwiches is sandwiches is bad for you!bad for you!

Before we Before we can decide can decide

how bad this how bad this is, we need is, we need

to know what to know what this is 20% this is 20%

of.of.

But why does it But why does it matter what matter what it’s 20% of?it’s 20% of?

20% is 20%, isn’t 20% is 20%, isn’t it?it?

What’s 20% of 100 people?What about 20% of 10 people?Or 20% of 150 people?

Colour the right Colour the right number of squares number of squares

on your on your worksheet.worksheet.

20% of 100 people = 2020% of 10 people = 220% of 150 people = 30

20% of 100 people = 2020% of 10 people = 220% of 150 people = 30

2 is a lot 2 is a lot fewer fewer people people than than

20 or 3020 or 30

The missing information …The missing information …

We need to know how many people will get bowel cancer even if they don’t even if they don’t eat bacon sandwicheseat bacon sandwiches to know whether we should worry about a 20% increase or not.If there’s a large risk anyway, then a 20% increase will be a lot more people.If there isn’t a large risk anyway, then a 20% increase will be a much smaller number of people.

The missing information …The missing information …

We need to know how many people will get bowel cancer even if they don’t even if they don’t eat bacon sandwicheseat bacon sandwiches to know whether we should worry about a 20% increase or not.If there’s a large risk anyway, then a 20% increase will be a lot more people.If there isn’t a large risk anyway, then a 20% increase will be a much smaller number of people.

The missing information …The missing information …

We need to know how many people will get bowel cancer even if they don’t even if they don’t eat bacon sandwicheseat bacon sandwiches to know whether we should worry about a 20% increase or not.If there’s a large risk anyway, then a 20% increase will be a lot more people.If there’s only a small risk anyway, then a 20% increase will be a much smaller number of people.

The risk that a person will get bowel cancer at some point in their life is about 1 in 18, which is roughly 5%.

This means that even if they never eat a bacon sandwich about 5 people in every 100 will get bowel cancer.

20% of 5 people is 1 person.

So an extra person can be expected to get bowel cancer if 100 people eat bacon sandwiches throughout their lives.

It is this risk – the absolute risk of getting bowel cancer – which is increased by 20% if someone eats a bacon sandwich every day.

The risk that a person will get bowel cancer at some point in their life is about 1 in 18, which is roughly 5%.

This means that even if they never eat a bacon sandwich about 5 people in every 100 will get bowel cancer.

20% of 5 people is 1 person.

So an extra person can be expected to get bowel cancer if 100 people eat bacon sandwiches throughout their lives.

On your worksheetcolour a random 5% of smiley faces on each grid to show people who will get bowel cancer regardless of eating bacon sandwiches

5% of 100 is 5

5% of 60 is 3

5% of 140 is 7

Using a different colouradd the extra cases of bowel cancer which would occur if all these people ate bacon sandwiches every day

An extra 20% of 5 is 1, giving a total of 6.

An extra 20% of 3 is 0.6, giving a total of 3 or 4.

An extra 20% of 7 is 1.4, giving a total of 8 or 9.

The missing information …The missing information …

A 20% increase sounds a lot, doesn’t it.

But because it’s 20% of a small absolute risk, it actually means that we should expect just one extra person to get bowel cancer in every 100 who eat bacon sandwiches every day.

What do you think? Will this stop you eating bacon sandwiches?

Motivate ProjectMillennium Mathematics Project

University of Cambridge

We are grateful to the Wellcome Trust

for funding this project.

© University of Cambridge